That's "extra" plastic that doesn't have anywhere to go when it extrudes. If this is your first layer, print slightly higher, and if this is a higher layer than that, try lowering your extrusion multiplier by 1 or 2% to see if that makes a difference.

You really should be setting your height with calipers and your multiplier by measuring a thin wall object, but a tweak here and there to fix a symptom like this isn't out of line.